[bug] size change callback from _MeasureSize not working with AnimatedSize

[daadu]

To Reproduce

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> with SingleTickerProviderStateMixin {
  String text = "Back Layer 1";
  double height = 100;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: "Backdrop Example",
      theme: ThemeData(
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: BackdropScaffold(
        appBar: BackdropAppBar(
          title: Text("Backdrop Example"),
          actions: <Widget>[
            LayoutBuilder(
              builder: (context, constraints) => IconButton(
                icon: Icon(Icons.terrain),
                onPressed: () {
                  setState(() {
                    height = 200;
                    text = "Back Layer 2";
                  });
                },
              ),
            ),
          ],
        ),
        stickyFrontLayer: true,
        backLayer: AnimatedSize(
          vsync: this,
          duration: Duration(milliseconds: 200),
          child: Container(
            height: height,
            child: Center(child: Text(text)),
          ),
        ),
        frontLayer: Center(
          child: Text("Front Layer"),
        ),
      ),
    );
  }
}

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots

Additional context
Reported by: @proninyaroslav

[daadu]

@proninyaroslav Test the fix against fix/measure_size branch.

[proninyaroslav]

Works great, thanks.